The present invention relates to a method for the overload protection of devices which cannot endure continuous running on account of heat loss, having a temperature-dependent resistor measuring the inside temperature of the device and having an evaluation circuit.
In the case of devices which may not be operated uninterruptedly, for example for reasons of heat development, an overloading of the devices must be prevented by suitable measures. For this purpose it is known from German Patent Document DE 32 48 217 A1 to measure the inside temperature of the device in the exhaust airstream of the device with a temperature-dependent resistor, which is part of an evaluation circuit and which, upon corresponding actuation, reduces the motor output to a lower value to avoid overheating.
Generally known measures for overload protection also comprise running time counters, bimetal switches or temperature measuring elements arranged directly in the device on its critical components reducing heat loss.
It may be considered disadvantageous in the case of these known measures that there is not optimum utilization of the devices--running time counter without reference to the temperature--or that there are high costs due to additional lines of the temperature measuring point and that the installation of a bimetal switch or a temperature measuring element directly in the device is complex.
An object of the invention is to provide a method for overload protection of devices which cannot endure continuous running on account of heat loss.
This and other objects are achieved by the present invention which provides a method for the overload protection of devices which cannot endure continuous running on account of heat loss, having a temperature-dependent resistor measuring the inside temperature of the device and having an evaluation circuit. The method includes measuring by means of the resistor arranged outside the device and remote from its device parts producing heat loss the inside temperature inside a housing enclosing the device. A temperature difference assigned to the inside temperature is read out from a device-specific characteristic stored in the evaluation circuit. The difference T.sub.u =.nu..sub.u -.DELTA.T is then formed in a subtracter of the evaluation circuit. From a further device-specific characteristic stored in the evaluation circuit a permissible operating period of the device is read out. This permissible operating period is assigned to the temperature difference. A preset operating period cycle time is determined using the permissible operating period as the basis for a preset operating period cycle time. After reaching the permissible operating period, the device is turned off by the evaluation circuit during the operating period cycle time.
The objects are also achieved by another embodiment of the present invention which provides a method for the overload protection of devices which cannot endure continuous running on account of heat loss, having a temperature-dependent resistor measuring the inside temperature of the device and having an evaluation circuit. The method includes measuring by means of the resistor arranged outside the device and remote from its device parts producing heat loss the inside temperature inside a housing enclosing the device. From a device-specific characteristic stored in the evaluation circuit, which characteristic is derived from device-specific characteristics, a permissible operating period of the device that is assigned to the inside temperature is read out. The permissible operating period is used as the basis for determining a preset operating period cycle time. After reaching the permissible operating period, the device is turned off by the evaluation circuit during the operating period cycle time.
In particular, with the method of the present invention, the device is utilized optimally with respect to its operating period cycle time and also, there is no need for a cost-intensive, direct coupling of the temperature measuring element in the device to its critical components producing heat loss.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.